That depends on the site. I don't think anyone can give you a clear answer.

You said it yourself. You will need to test and find out... Maybe you can use both... Maybe amazon will also be a good idea... Test Test Test.

Generally, if you are getting a low click through rate for AdSense, CPA will be no better - unless you can find something ultimately targeted at your audience.

CPA can pay more, but you have to factor in the lower conversion rate as well. Give it a shot - it isn't hard to change back.

If your adsense conversion is low then the CPA conversion will be low as well,
maybe you could try CPM if you have good traffic